TEKASHI 6IX9INE SUED FOR BAILING ON CONCERT ... $300K On The Line

Tekashi 6ix9ine's legal woes continue to stack up as the rapper sits behind bars ... he's now being sued for bailing on a concert and costing promoters big bucks.

6ix9ine was supposed to play a show on Howard Homecoming Weekend back in October. According to the docs -- obtained by TMZ -- the rapper signed on to do the show for $60,000 ... but according to promoters, never showed up and forced them to turn away thousands of fans.

The docs state concert organizers were scrambling to get in touch with Tekashi on the date of the show ... and his management team -- who are also listed as defendants in the lawsuit -- did little to help.

Promoters say tons of fans showed up at the venue and waited outside for hours for the doors to open. It wasn't until 8 PM -- when concertgoers were supposed to be admitted to the event -- that organizers had to let everyone know the show was canceled ... fearing a riot would break out.

Organizers claim they sold $165,336 in presale sears and another $30,300 in VIP tables for the event.

The docs -- filed by attorneys Jelani D. James and Donald M. Temple -- claim no less than $300k in damages.